Irenaeus of Lyon, writing around 180 CE against what he calls Valentinian heresy, mentions in passing that the group possesses its own Gospel of Truth, differing greatly from the four gospels the church received from the apostles. He gives no summary of its contents and does not say who wrote it beyond attributing it to the followers of Valentinus. For eighteen centuries that sentence was the entire record of the work's existence.
In 1945 a jar of Coptic codices was found near Nag Hammadi in Upper Egypt, and one of them, later called the Jung Codex after the psychologist whose institute in Zurich acquired it in 1951 as a gift, opens with a text that calls itself, in its own first lines, the gospel of truth. Whether this is the same composition Irenaeus meant is not something the text itself settles, since it never repeats that phrase as a formal title elsewhere and Irenaeus quotes nothing from it that can be matched word for word against the Coptic. Scholars who find the general Valentinian character of the two descriptions close enough treat the identification as likely; those who note how little Irenaeus actually says treat it as unproven rather than false.
The same uncertainty carries into authorship. Harold Attridge and George MacRae, introducing the standard English translation, observe that the text's style and ideas sit close enough to the handful of fragments known to be Valentinus's own writing that he remains a real candidate as author, though they stop short of calling this established. Irenaeus's own attribution, to Valentinus's followers rather than to Valentinus himself, is the more cautious position and the one most scholars still hold. Both readings agree the work is Valentinian; they disagree about how close it stands to the school's founder.